Rate adaptation in a non-uniform non-transparent data channel

ABSTRACT

Rate adaptation method and equipment in a telecommunication system for data transmission employ a frame-structured data transmission protocol between the telecommunication equipment of the transmitting party and the telecommunication equipment of the receiving party via a non-uniform non-transparent traffic channel or set of traffic channels consisting of a traffic channel section of a higher capacity and a traffic channel section of a lower capacity. The interface between the high capacity and the low capacity traffic channel section is provided with rate adaptation equipment that consists of a data buffer and data buffer control. The data buffer buffers the frames received from the high capacity traffic channel section before they are transmitted to the low capacity traffic channel section. The data buffer control adjusts the filling level of the data buffer by activating, if necessary, data flow control according to the data transmission protocol in the telecommunication equipment of the transmitting party.

This application is the national phase of international applicationPCT/FI96/00654, filed Dec. 10, 1996 which designated the U.S.

FIELD OF THE INVENTION

The invention relates to a rate adaptation method and a rate adapter fora non-uniform non-transparent data transmission channel, especially inmobile systems having a non-standard transmission capacity at the radiointerface

BACKGROUND OF THE INVENTION

Mobile systems generally refer to various telecommunication systems thatenable private wireless data transmission for subscribers moving withinthe system. A typical mobile system is a public land mobile network(PLMN). The PLMN comprises fixed radio stations (base stations) locatedin the service area of the mobile network, the radio coverage areas(cells) of the base stations providing a uniform cellular network. Abase station provides in the cell a radio interface (air interface) forcommunication between a mobile station and the PLMN. Since mobilestations can move in the network and they have access to the PLMNthrough any base station, the PLMNs are provided with complicatedarrangements for subscriber data management, authentication and locationmanagement of mobile subscribers, for handovers (a change of a basestation during a call) etc. The networks are also provided with servicesthat support the transmission of information other than the usual speechcalls (speech service), such as data, facsimile, video image, etc. Thesenew services have required a considerable amount of developmental workand new arrangements in the networks.

Another area of mobile systems includes satellite-based mobile services.In a satellite system, radio coverage is obtained with satellitesinstead of terrestrial base stations. The satellites are located on anorbit circling the earth and transmitting radio signals between mobilestations (or user terminals UT) and land earth stations (LES). The beamof the satellite provides on the earth a coverage area, i.e. a cell. Thecoverage areas of individual satellites are arranged to form continuouscoverage so that a mobile station is located at all times within thecoverage area of at least one satellite. The number of the satellitesneeded depends on the desired coverage. Continuous coverage on thesurface of the earth might require for example 10 satellites.

Subscriber mobility requires similar arrangements in satellite mobilesystems as in the PLMNs, i.e. subscriber data management, authenticationand location management of mobile subscribers, handovers, etc. Thesatellite systems should also support similar services as the PLMNs.

One way of implementing these requirements in satellite mobile systemsis to use existing PLMN arrangements. In principle this alternative isvery simple since a satellite system can be basically compared to a basestation system of a mobile system having an incompatible radiointerface. In other words, it is possible to use a conventional PLMNinfrastructure where the base station system is a satellite system. Insuch a case, the same network infrastructure could in principle evencontain both conventional PLMN base station systems and satellite "basestation systems".

There are many practical problems related to the adaptation of the PLMNinfrastructure and a satellite system, however. A problem apparent tothe Applicant is that a PLMN traffic channel and a traffic channel of a"radio interface" in a satellite system differ considerably. Examine anexample where the PLMN is the Pan-European digital mobile system GSM(Global System for Mobile Communication) and the satellite mobile systemis the Inmarsat-P system that is currently under development.

A traffic channel in the GSM system supports data transmission at theuser rates of 2400, 4800, 7200 and 9600 bit/s. In the future, high-speeddata services (HSCSD=High speed circuit switched data) employing two ormore traffic channels at the radio interface (multi-slot access) alsosupport higher user rates (14400 bit/s, 19600 bit/s, . . . ).Non-transparent data services also utilize a radio link protocol RLPbetween a mobile station MS and an interworking function IWF, which istypically situated at a mobile services switching centre MSC. The RLP isa frame-structured balanced (HDLC-type) data transmission protocol.Error correction in the RLP is based on the retransmissions of framescorrupted on the traffic channel, requested by the receiving party. Thetraffic channel employs channel coding that aims at decreasing theeffect of transmission errors. Due to the channel coding and the otheroverhead information the bit rate at the radio interface will be higherthan the actual user rate. The radio interface rates for the user ratesof 2400, 4800 and 9600 bit/s are 3600, 6000 and 12000 bit/s,respectively.

The Inmarsat-P satellite system requires that standard data rates up to4800 bit/s can be transmitted on one traffic channel (e.g. 1200, 2400,4800 bit/s) and that standard data rates exceeding 4800 bit/s (e.g.9600, 14400, 19200 bit/s, etc.) can be transmitted by using severalparallel traffic channels, such as in the HSCSD service of the GSMsystem.

In the Inmarsat-P satellite system, the data rate of one traffic channelat the radio interface is at most 4800 bit/s, which equals the user datarate of 4800 bit/s at the terminal interface. In a data serviceemploying two traffic channels the data rate at the radio interfaceequals the user data rate of 9600 bit/s at the terminal interface. Inother words, an end-to-end traffic channel between an MS and an MSC isnon-uniform since the capacity of the traffic channel section over thesatellite leg is lower than that of the traffic channel section betweenan LES and the MSC. This non-uniformity of the traffic channel causesthe following problems in non-transparent data services employing theRLP protocol.

Firstly, the MSC-IWF transmits data towards the MS at the same rate asdata is received from the fixed network, such as the ISDN or the PSTN.In practice, this may signify a data rate of 12000 bit/s in anon-transparent call since the data modem of the IWF may operate in anautobauding mode towards the fixed network. The LES may transmit datathat it receives from the MSC-IWF towards the MS at a considerably lowerrate, i.e. 4800 bit/s. The MS may easily receive the data that the LEStransmits with the lower data rate, but the data begins to accumulateand may be lost at the LES. However, the MSC-IWF continues transmittingat the full rate of 12000 bit/s until a transmission window set in theRLP protocol is full. A transmission window refers to the number of theRLP frames the transmitting party may transmit without receivingacknowledgment from the receiving party.

A similar problem may also occur when other types of radio interfacesare connected to PLMNs, for example wireless telephone systems where thecapacity of the traffic channel section at the radio interface is lowerthan that of the traffic channel section in the remaining part of thePLMN.

SUMMARY OF THE INVENTION

An object of the present invention is to eliminate or alleviate theabove-described problem.

The present invention relates to rate adaptation equipment in atelecommunication system for data transmission employing aframe-structured data transmission protocol between thetelecommunication equipment of the transmitting party and thetelecommunication equipment of the receiving party via a non-uniformnon-transparent traffic channel or set of traffic channels consisting ofa traffic channel section of a higher capacity and a traffic channelsection of a lower capacity. The equipment is characterized in that therate adaptation equipment is located at an interface between the highcapacity and the low capacity traffic channel section and comprises

a data buffer that buffers the frames that are received from the highcapacity traffic channel section before they are transmitted to the lowcapacity traffic channel section,

data buffer control arranged to adjust the filling level of the databuffer by activating, if necessary, data flow control according to saiddata transmission protocol in the telecommunication equipment of thetransmitting party.

The invention also relates to a rate adaptation method for datatransmission employing a frame-structured data transmission protocolbetween the telecommunication equipment of the transmitting party andthe telecommunication equipment of the receiving party via a non-uniformnon-transparent traffic channel or set of traffic channels consisting ofa traffic channel section of a higher capacity and a traffic channelsection of a lower capacity. The rate adaptation method is characterizedin that it comprises the steps of

buffering, in the data buffer, the frames that are received from thehigh capacity traffic channel section before they are transmitted to thelow capacity traffic channel section,

monitoring the filling level of the data buffer,

intervening on the data transmission protocol between thetelecommunication equipments and activating data flow control accordingto the data transmission protocol at the transmitting party when thefilling level of the data buffer increases to the first threshold value,

deactivating the data flow control according to the data transmissionprotocol in the direction of the transmitting party when the fillinglevel of the data buffer decreases to the second threshold value and theactivation of the data flow control is not requested by the receivingparty.

In the present invention, between the traffic channel section of ahigher capacity and the traffic channel section of a lower capacitythere is (e.g. in the downlink direction from the mobile network towardsan MS) a data buffer which provides the required rate adaptation bystoring data frames received from the high capacity traffic channelsection until they can be transmitted via the low capacity trafficchannel section. For example, in association with the radio linkprotocol RLP, the most simple approach would be to design the databuffer to be able to store one RLP window of RLP frames received fromthe high capacity traffic channel section. This approach would beoperative under certain circumstances, but it involves severaldrawbacks. 1) The long buffering causes a delay that results in theexpiration of the RLP retransmission timer at the transmitting endunless the timer has been set to a sufficiently long value in thenegotiation at the beginning of the data session. 2) On the other hand,if the timeout of the retransmission timer has been negotiated to avalue that is sufficiently long in order to prevent unnecessaryretransmissions, the actual error correction, which is based on theretransmission of the frames as the retransmission timer expires, willbe slower. It further slows down when the retransmitted frames (as wellas the frames that have been transmitted for the first time) must stillstay in the retransmission queue at the LES. 3) Additional memorycapacity is required at the base station or at an LES of the satellitesystem. 4) The problems become worse when a high rate multichannelconnection is used. In such a case, the larger RLP window requires alonger data buffer and a longer retransmission timer (up to eight timeslonger, depending on the number of channels over the connection). Over amultichannel connection, this problem may also occur in the otherdirection (from the MS towards the network) if the radio interfacecapacity of the multiple channel connection is higher than thetransmission channel capacity between a base station or a land earthstation and the rest of the PLMN.

Therefore in the preferred embodiment of the invention, the status ofthe aforementioned data buffer is monitored and the data transmissionprotocol, e.g. the radio link protocol that operates between thetransmitting party and the receiving party, is interrupted and RLP flowcontrol is activated towards the transmitting RLP party. In other words,the data buffer is arranged to buffer data that is received from thehigh capacity traffic channel section and to transmit data to the lowcapacity traffic channel at the data rate supported by the latter one.In the normal mode, the RLP frames are forwarded through the data bufferwithout modification. If the data buffer fills up to a preset thresholdvalue, the unit that controls the data buffer, for example a basestation or an LES of the satellite system, interrupts the radio linkprotocol between the transmitting and the receiving party and initiatesan RLP flow control mode. This is achieved by transmitting a "receivernot ready (RNR)" radio link protocol frame towards the transmittingparty. The transmitting party stops transmitting the RLP frames when theflow control mode is active. In such a case, no more frames are receivedat the data buffer but it can be emptied with a rate enabled by the lowcapacity traffic channel. On the other hand, when the flow control modeis active, "receiver ready (RR)" frames possibly obtained from thereceiving party cannot be forwarded to the transmitting party since thiswould cancel the flow control mode. The unit controlling the data buffermust convert the possible RR frames into RNR frames so that theacknowledgment procedure of the RLP frames would not be disturbed. Theunit controlling the data buffer also monitors the RLP frames, which aretransmitted from the receiving party to the transmitting party, in orderto follow the acknowledgment situation of the frames and to detectwhether the receiving party transmits an RNR frame for some otherreason. When the data buffer is emptied to the lower threshold value,the unit controlling the data buffer checks whether the receiving partyhas transmitted an RNR frame that it has not cancelled with a subsequentRR frame. If such an RNR mode initiated by the receiving party is notactive, the unit controlling the data buffer again intervenes on the RLPprotocol between the receiving and the transmitting party and transmitsto the transmitting party an RR frame, i.e. it cancels the RLP flowcontrol mode. If the RNR mode that the receiving party has initiated isactive, the unit controlling the data buffer does not interrupt the RLPprotocol but it prepares to let all the subsequent RR frames pass fromthe receiving party to the transmitting party.

The invention overcomes the problem of rate adaptation in a non-uniformtraffic channel where the different sections of the connection havedifferent data transmission capacities and where an error correctionprotocol is used throughout this non-uniform traffic channel. Theinvention prevents the excessive accumulation of data at the interfaceof the traffic channel sections by using buffering which can bemaintained rather minor, however, by monitoring the filling level of thebuffer and by initiating, if required, a flow control mode at thetransmitting end.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described by means of preferredembodiments with reference to the accompanying drawings, in which

FIG. 1 is a block diagram illustrating a configuration according to theGSM recommendations for data transmission,

FIG. 2 shows the structure of an RLP frame,

FIG. 3 shows the format of a header in an RLP frame,

FIG. 4 is a block diagram illustrating how the Inmarsat-P satellitesystem is connected as a base station system to a GSM-based mobilesystem,

FIG. 5 is a functional block diagram illustrating the buffer equipmentaccording to the invention,

FIG. 6 is a flow chart illustrating the operation of the bufferequipment of FIG. 5.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention can be applied for data transmission through anytraffic channel consisting of two or more sections of differenttransmission rates. The preferred embodiments of the invention will bedescribed by using as an example the interworking between a GSM-basedmobile system and the Inmarsat-P satellite system connected thereto as a"base station system". However, the invention is not to be restricted tothese systems.

The structure and operation of the GSM mobile system are well known to aperson skilled in the art and they are defined in the GSM specificationsof the ETSI (European Telecommunications Standards Institute). Referenceis also made to GSM System for Mobile Communication by M. Mouly and M.Pautet (Palaiseau, France, 1992, ISBN:2-9507190-0-7). GSM-based mobilesystems include DCS1800 (Digital Communication System) and the USdigital cellular system PCS (Personal Communication System).

The configuration according to the GSM recommendations for datatransmission is illustrated in FIG. 1. The basic structure of the GSMmobile system is shown in FIG. 1. The GSM structure comprises two parts:a base station system BSS 100 and a network subsystem NSS. The BSS 100and the MSs 102 communicate via radio connections. In the BSS, each cellis serviced by a base station BTS (not shown in the figure). A number ofbase stations are connected to a base station controller BSC (not shownin FIG. 1) the function of which is to control the radio frequencies andchannels used by the BTS. The BSSs are connected to an MSC 104. CertainMSCs are connected to other telecommunication networks, such as thepublic switched telephone network PSTN and the ISDN 106.

In the GSM system, a data connection 108 is established between aterminal adaptation function TAF of an MS and an IWF in the mobilenetwork (usually in the MSC). In data transmission occurring in the GSMnetwork, this connection is a V.110 rate-adapted, UDI-coded digitalfull-duplex connection that is adapted to V.24 interfaces. The V.110connection described herein is a digital transmission channel that wasoriginally developed for ISDN (Integrated Services Digital Network)technology, that is adapted to the V.24 interface, and that alsoprovides the possibility of transmitting V.24 statuses (controlsignals). The CCITT recommendation for a V.110 rate-adapted connectionis disclosed in the CCITT Blue Book: V.110. The CCITT recommendation fora V.24 interface is disclosed in the CCITT Blue Book: V.24. The TAF 108adapts the data terminal TE 110 connected to the MS to theaforementioned GSM V.110 data connection which is established over aphysical connection utilizing one or several traffic channels (HSCSD).The IWF comprises a rate adapter that adapts the GSM V.110 dataconnection to the V.24 interface, and a data modem or another rateadapter depending on whether the connection is extended to the PSTN orthe ISDN. In the ISDN or the PSTN, a data connection is established forexample to another TE. The V.24 interface between the MS and the TE iscalled here a terminal interface. A corresponding terminal interface isalso located in the IWF as well as in the other TE 112 in the ISDN orthe PSTN.

A GSM traffic channel supports data transmission with the user rates of2400, 4800, 7200 and 9600 bit/s. In the future, high-speed data services(HSCSD=High speed circuit switched data) employing two or more trafficchannels at the radio interface (multi-slot access) also support higheruser rates (14400 bit/s, 19600 bit/s, . . . ). The traffic channelemploys channel coding that aims at decreasing the effect oftransmission errors. The channel coding and the various overheadinformation increase the bit rate at the radio interface higher than theactual user rate. The radio interface rates corresponding to the userrates of 2400, 4800 and 9600 bit/s are 3600, 6000 and 12000 bit/s.

Non-transparent data services also employ a radio link protocol RLPbetween an MS and an IWF. The IWF is typically situated at the MSC. TheRLP is a frame-structured balanced (HDLC-type) data transmissionprotocol that is described in the GSM recommendation 04.22. Moreprecisely, data is transmitted between the TAF and the IWF in the RLPframes of FIG. 2. FIG. 3 shows the format of the header field. Theparameters of the header are disclosed in the GSM recommendation 04.22.The RLP frame has a fixed length of 20 bits that consists of a header(16 bits) and an information field (200 bits) and a frame check sequence(FCS) (24 bits). The error correction in the RLP is based on theretransmission of frames corrupted on the traffic channel, requested bythe receiving party. Therefore the transmitted RLP frames are stored inthe transmitting end until an acknowledgment is received from thereceiving end. If the transmitting end does not receive anacknowledgment or receives a negative acknowledgment, the transmittingend repeats the transmission a predetermined number of times. The numberof retransmissions is restricted by a parameter N2 so that endlesstransmission loops due to a very poor connection are avoided. Thetransmission of the RLP frame is repeated when a preset time T1 haselapsed since the preceding transmission. In order to decrease theamount of buffering required in the transmitting end, a sliding windowis used for acknowledgment. This means that the transmitting party maytransmit several frames before an acknowledgment is required from thereceiver. The RLP window therefore represents a sliding group of framesthat have already been transmitted but that have not been acknowledged.The size of the window (the number of frames) is determined by aparameter W. The RLP parties negotiate the values of the parameters N2,T1 and WS at the beginning of the session.

The RLP protocol also comprises flow control which is used to adjust thefilling level of the transmission and reception buffers in the TAF andthe IWF. This flow control is described for example in the GSMrecommendation 07.02. The receiving RLP party (TAF or IWF) immediatelytransmits a "flow control active" indication to the transmitting party(IWF or TAF) if the reception buffer from the radio path reaches apredetermined threshold value, or if the TE has initiated local flowcontrol at the terminal interface. In the last case, when the TAFreceives from the terminal interface this flow control indication, itinterrupts the data transmission from the reception buffer to theterminal interface. When the buffer disabled mode or the local flowcontrol is eliminated, the TAF/IWF transmits a "flow control inactive"indication. Also, the data transmission from the reception buffer isreactivated. The transmitting party activates the local flow controlimmediately when the transmission buffer reaches a predeterminedthreshold value or when it receives a "flow control active" indicationfrom the receiving party. When the buffer disabled mode is terminated orwhen the "flow control inactive" indication is received, the IWF/TAFremoves the local flow control. The transmission buffer buffers the datareceived from the V.24 interface in such a way that data is not lost ifthe MS is not able to transmit data immediately over the radio path. Thereception buffer buffers the data that is transmitted to the V.24interface in such a way that the data received from the traffic channelis not lost if it cannot be immediately forwarded via the V.24interface, for example to the TE. The aforementioned predeterminedthreshold value that activates the flow control is for example atransmission or a reception buffer that is half full.

The "flow control active" indication is an RLP frame that comprises inthe header field "RNR (receiver not ready)" coding, i.e. bits S1S2=10.This frame is called an RNR frame. The "flow control inactive"indication is an RLP frame comprising in the header field "RR (receiverready)" coding, i.e. bits S1S2=00. This frame is called an RR frame.

In FIG. 4, the Inmarsat-P satellite system 114 is connected as a basestation system to a GSM-based mobile system. In the Inmarsat satellitesystem, radio coverage is obtained by satellites instead of basestations located on the ground, the satellites being on an orbitcircling the earth and transmitting radio signals between MSs (or userterminals UT) and LESs 116. The beam of the satellite forms a coveragearea, i.e. a cell, on the earth. The coverage areas of individualsatellites are arranged to form continuous coverage so that an MS is atall times located within the coverage area of at least one satellite.The number of the satellites required depends on the desired coverage.Continuous coverage on the surface of the earth might require forexample 10 satellites. FIG. 4 shows, for the sake of clarity, only oneLES 116, one satellite SAT 114 and one MS 102. The LES is connected tothe MSC of the GSM network in the same way as the BSS in FIG. 1. Alsothe GSM protocols between the MSC and the LES are the same as betweenthe MSC and the BSS in FIG. 1 (GSM V.110). The terminal interface isalso the same as in FIG. 1. The difference is that in FIG. 4 the GSMV.110 connection is not used over the entire connection between the MSCand the MS but the radio interface between the LES and the MS uses theInmarsat protocols and traffic channels.

A radio interface consists of a bidirectional satellite radio connectionbetween an MS and an LES. The exact structure or operation of the unitsSAT, LES and MS in the satellite system or the accurate specificationsof the radio interface are not relevant to the present invention. Theinvention does not require changes in the actual satellite system thedetails of which can be obtained from the Inmarsat specifications. Theonly feature essential to the invention is the capacity of the trafficchannel formed over the radio interface. In other words, in theInmarsat-P system the maximum data rate of a traffic channel is 4800bit/s, whereas in the GSM network section the maximum data rate of thetraffic channel is 12000 bit/s. Therefore the end-to-end traffic channelbetween the TAF and the IWF is non-uniform which causes theabove-described problem, i.e. the accumulation of data at the LES.

A similar problem may also occur in other systems where the trafficchannels are non-uniform, i.e. the traffic channel capacity is differentat the radio interface than in the other parts of the network.

This is overcome according to the invention by providing a network unit,such as an LES of a satellite system or a BTS of a radio system, whichis situated at the interface of traffic channel sections of differentcapacities, with a data buffer that provides the required rateadaptation. The degree of use of the data buffer is monitored andadjusted by utilizing the end-to-end RLP protocol that operates betweenthe transmitting and the receiving party. This is performed byactivating the RLP data flow control in the direction of thetransmitting RLP party by means of RNR frames when the buffer is fulland by deleting the data flow control activation by means of RR frameswhen the buffer is empty. It should be noted that the present inventiondoes not require any changes in either RLP unit, such as an IWF and aTAF. The exact implementation of the units is therefore not essential tothe invention and will not be described in greater detail in the presentapplication. Instead, the present invention utilizes the existingfeature of the RLP protocol, i.e. data flow control. The data flowcontrol according to the invention is based entirely on a new type ofuse of RR and RNR frames. The transmitting and the receiving RLP unitare not aware of the interruption of the end-to-end protocol and the useof the frames on the traffic channel. Instead, they assume that theyinterchange frames according to the protocol directly between them.Therefore it is only necessary to describe the operation according tothe invention carried out by the network unit situated between theparties in order to understand and implement the present invention.

FIG. 5 illustrates the implementation of the invention at an LES of asatellite system. FIG. 5 only shows the functional blocks that areessential to the invention. In FIG. 5, the downlink direction is thetransmission direction from the IWF to the TAF and the uplink directionis the opposite direction. The LES comprises a data buffer 51 thatreceives RLP frames from the IWF via a GSM traffic channel having therate of 12000 bit/s. The data buffer 51 buffers the received RLP framesin order to transmit them to the TAF via an Inmarsat-P traffic channelat the rate of 4800 bit/s. The LES also comprises data buffer modecontrol that adjusts the filling level of the data buffer 51 in themanner according to the invention. The LES also comprises a unit 53 formonitoring and processing the frames, the unit receiving the uplink RLPframes from the TAF at the rate of 4800 bit/s. The unit 53 forwards theinformation about the frame type to the control unit 52 and transmitsthe frames to the IWF at the rate of 12000 bit/s either as such or in amodified form, as will be described in greater detail below.

The operation of the equipment of FIG. 5 will be described below withreference to FIG. 6. FIG. 6 is a flow chart illustrating the operationof the unit 52 in the mode control of the data buffer 51.

Assume first that the data buffer 51 is given a higher threshold valueTHRESH1 and a lower threshold value THRESH2 that describe the fillinglevel of the data buffer. Each threshold value corresponds to a presetnumber of RLP frames in the data buffer, for example. Assume also thatthe data buffer 51 is in the normal operating mode. In other words, thefilling level of the data buffer is between THRESH1 and THRESH2, and thedata flow control according to the invention is not active.

With reference to FIG. 6, the control unit 52 monitors the uplink RLPframes that the TAF transmits to the IWF. More particularly, the framemonitoring and processing unit 53, for example buffer storage, capturesthe frame, reads the frame number and forwards the frame number to thecontrol unit 52. The control unit 52 stores in its memory the latestacknowledged frame number. In this manner, the control unit 52 is ableto monitor the acknowledgment situation of the frames.

The control unit 52 also monitors the filling level of the data buffer51 (steps 63 and 64). If the filling level of the buffer has not reachedthe higher threshold value THRESH1, the process moves from step 64 backto step 62. If the data buffer 51 has filled up to the threshold valueTHRESH1, the control unit 52 intervenes on the RLP protocol between theTAF and the IWF and activates data flow control in the IWF. This takesplace in such a way that the control unit 52 commands the framemonitoring and processing unit 53 to transmit an RNR frame comprisingthe latest acknowledged frame number to the IWF. When the IWF receivesthe RNR frame it stops transmitting data frames in the downlinkdirection to the data buffer 51 and activates the local data flowcontrol according to the GSM recommendation 07.02. However, the databuffer 51 continues transmitting the RLP frames in the downlinkdirection to the TAF, which results in the data buffer being graduallyemptied. The TAF simultaneously continues transmitting the RLP frames inthe uplink direction according to the RLP protocol. In order to avoiderror situations, the control unit 52 must monitor whether the uplinkframes comprise RR or RNR frames. For this purpose, the frame monitoringand processing unit 53 buffers each uplink frame until the control unit52 has analyzed and possibly modified it.

If the control unit 52 detects that the uplink frame is an RR frame(step 66), it converts the frame into an RNR frame (step 67) and resetsthe RNR flag (step 68). The purpose of this conversion of the frame typeis to prevent the RR frames from eliminating the local data flow controlin the IWF before the data buffer 51 has been emptied sufficiently. Theprocedure then moves on to step 71.

If the result of the analysis in step 66 was that the uplink frame isnot an RR frame, the control unit 52 analyzes in step 69 whether it isan RNR frame. If the frame is an RNR frame, an RNR flag, which ismaintained by the control unit 52 in its memory, is set (step 70). TheRNR flag indicates that the TAF itself has transmitted the RNR frame inorder to activate the local flow control in the IWF. The RNR frame isset until the next RR frame resets it in step 68.

The process proceeds from step 70 to step 71. Step 69 is also followeddirectly by step 71 if the uplink frame is not an RNR frame. In step 71,the frame number in the memory of the control unit 52 is updated and theuplink frame is forwarded to the IWF.

In step 72, the control unit 52 checks whether the filling level of thedata buffer 51 has decreased to the lower threshold value THRESH2. If ithas not, the process returns to step 66. If it has, it is checked instep 73 whether the RNR flag has been set. If it has not been, an RRframe provided with the latest acknowledged frame number is transmittedto the IWF (step 74). This terminates the local data flow control thatwas activated by the control unit 52 in order to empty the data buffer51. The process then returns to step 62.

If it is detected in step 73 that the RNR flag has been set, itindicates that the TAF has also activated the local data flow control inthe IWF. Therefore the process moves directly from step 73 to step 62 sothat the data flow control in the IWF continues until the TAF transmitsan RR frame.

The invention has been described above applied in the downlinkdirection. Corresponding rate adaptation may also be needed in theuplink direction for example in connection with HSCD services. In such acase, it is possible to allocate at the radio interface such a number oftraffic channels that their total data rate is higher than the maximumdata rate of 12000 bit/s for one GSM traffic channel. The trafficchannels of the radio interface thus form the aforementioned trafficchannel section of a higher capacity and the GSM traffic channel formsthe traffic channel section of a lower capacity. The result is that theuplink RLP frames accumulate at the LES. This problem is eliminated inthe same manner as described above in the downlink direction. In thiscase, the local data flow control is activated in the TAF and the framestransmitted by the IWF are monitored and processed.

In general, the figures and the description related thereto are onlyintended to illustrate the present invention. The details of theinvention may vary within the scope and spirit of the appended claims.

I claim:
 1. Rate adaptation equipment in a telecommunication system fordata transmission employing a frame-structured end-to-end datatransmission protocol between telecommunication equipment of atransmitting party and telecommunication equipment of a receiving partyover a non-uniform non-transparent traffic channel or set of trafficchannels consisting of a traffic channel section of a higher capacityand a traffic channel section of a lower capacity, the rate adaptationequipment being located at an interface between the higher capacity andthe lower capacity traffic channel sections and comprising:a data bufferthat buffers frames received from the higher capacity traffic channelsection before the frames are transmitted to the lower capacity trafficchannel section; and data buffer control arranged to adjust a fillinglevel of the data buffer by intervening on the frame-structuredend-to-end transmission protocol and activating, based on the fillinglevel of the data buffer, data flow control according to said datatransmission protocol over said traffic channel section of highercapacity in the telecommunication equipment of the transmitting party,said data buffer control being arranged to pass the end-to-end protocolthrough the rate adaptation equipment without intervention otherwise,wherein: the data buffer control is arranged to convert acknowledgementframes of a second frame type, transmitted by the receiving party, intoacknowledgement frames of a first frame type before forwarding theacknowledgement frames to the transmitting party when the data flowcontrol is activated.
 2. Rate adaptation equipment according to claim 1,wherein the telecommunication system comprises a mobile network having aradio interface formed by a satellite system, and whereinthe satellitesystem comprises at least one land earth station connected as a basestation system to a mobile services switching centre of the mobilenetwork in such a way that a traffic channel of the mobile network,providing said high capacity traffic channel section, is used betweenthe land earth station and the mobile services switching centre, theradio interface is provided between the land earth station and a mobilestation through a satellite repeater, said radio interface employing thetraffic channel of the satellite system which provides said low capacitytraffic channel section, and the rate adaptation equipment is located atthe land earth station of the satellite system.
 3. Rate adaptationequipment according to claim 1, wherein the telecommunication systemcomprises a mobile network having a radio interface provided by asatellite system, andthe satellite system comprises at least one landearth station connected as a base station system to a mobile servicesswitching centre of the mobile network in such a way that a trafficchannel or a set of traffic channels of the mobile network, providingsaid low capacity traffic channel section, is employed between the landearth station and the mobile services switching centre, the radiointerface is provided between the land earth station and a mobilestation through a satellite repeater, said radio interface employing aset of traffic channels of the satellite system, said set of trafficchannels providing said high capacity traffic channel section, and therate adaptation equipment is located at the land earth station of thesatellite system.
 4. Rate adaptation equipment according to claim 1,wherein the first and the second telecommunication equipment comprise aterminal adaptation function of the mobile station and an interworkingfunction of the mobile network.
 5. Rate adaptation equipment accordingto claim 4, wherein the data transmission protocol is a radio linkprotocol, such as a radio link protocol according to the GSMrecommendation 04.22.
 6. Rate adaptation equipment according to claim 5,wherein said local flow control is flow control according to the GSMrecommendation 07.02.
 7. Rate adaptation equipment in atelecommunication system for data transmission employing aframe-structured end-to-end data transmission protocol betweentelecommunication equipment of a transmitting party andtelecommunication equipment of a receiving party over a non-uniformnon-transparent traffic channel or set of traffic channels consisting ofa traffic channel section of a higher capacity and a traffic channelsection of a lower capacity, the frame-structured data transmissionprotocol comprising a data flow control that is activated in thetelecommunication equipment of the transmitting party in response to aframe of a first frame type transmitted by the receivingtelecommunication equipment, and it is terminated in response toreceiving a frame of a second frame type, the rate adaptation equipmentbeing located at an interface between the higher capacity and the lowercapacity traffic channel sections and comprising:a data buffer thatbuffers frames received from the higher capacity traffic channel sectionbefore they are transmitted to the lower capacity traffic channelsection; a data buffer control being arranged to monitor the fillinglevel of the data buffer and acknowledgment frames that the receivingparty transmits to the transmitting party, the data buffer control beingarranged to intervene on said data transmission protocol between thetelecommunication equipment and to activate said local data flow controlin the telecommunication equipment of the transmitting party bytransmitting a frame of a first frame type when the filling level of thedata buffer increases to a first threshold value, the data buffercontrol being arranged to convert acknowledgement frames of a secondframe type, transmitted by the receiving party, into acknowledgementframes of the first frame type before they are forwarded to thetransmitting party when said local flow control is active, and the databuffer control being arranged to eliminate said activation of data flowcontrol by transmitting a frame of the second frame type when thefilling level of the data buffer decreases to a second threshold value.8. Rate adaptation equipment according to claim 7, wherein:the databuffer control is arranged to register the latest frame number that thereceiving party has acknowledged with an acknowledgement frame, and thedata buffer control is arranged to insert the registered latestacknowledgement number into the transmitted frames in order to activateor eliminate the data flow control.
 9. Rate adaptation equipmentaccording to claim 7, wherein:the data buffer control is arranged not toeliminate the activation of the data flow control even though thefilling level of the data buffer decreases to said second thresholdvalue, if the receiving party has transmitted, during the data flowcontrol activated by the data buffer control, a frame of the first frametype which has not been acknowledged by means of a frame of the secondframe type.
 10. A rate adaptation method for data transmission employinga frame-structured end-to-end data transmission protocol betweentelecommunication equipment of a transmitting party andtelecommunication equipment of a receiving party via a non-uniformnon-transparent traffic channel or set of traffic channels consisting ofa traffic channel section of a higher capacity and a traffic channelsection of a lower capacity, said rate adaptation methodcomprising:buffering, in a data buffer, frames received from the highercapacity traffic channel section before they are transmitted to thelower capacity traffic channel section, monitoring a filling level ofthe data buffer, intervening on the end-to-end data transmissionprotocol between the telecommunication equipment and activating dataflow control according to the data transmission protocol towards thetraffic channel section of higher capacity in the direction of thetransmitting party when the filling level of the data buffer increasesto a first threshold value, sending frames from the data buffer over thetraffic channel section of lower capacity to the receiving party, whilethe data flow control towards the traffic channel section of highercapacity is activated, deactivating the data flow control according tothe data transmission protocol towards the traffic channel section ofhigher capacity in the direction of the transmitting party when thefilling level of the data buffer decreases to a second threshold valueand the activation of the data flow control is not requested by thereceiving party, and converting acknowledgement frames of a second frametype, transmitted by the receiving party, into acknowledgement frames ofa first frame type before forwarding the acknowledgement frames to thetransmitting party when the data flow control is activated.